The prediction of the in vivo digestibility of the diet of sheep and cattle grazing indigenous hill plant communities by in vitro digestion, faecal nitrogen concentration or indigestible' acid‐detergent fibre

Herbage and faeces samples were retained from an intake and in vivo digestibility trial using material harvested at three different stages from five indigenous hill plant communities (those dominated by Agrostis‐Festuca., Nardus stricta., Molinia caerulea., Eriophorum vaginatum and Tri‐chophorum caespitosum) and from sown swards of ryegrass or white clover. Samples of the herbages and of their separate components, together with extrusa samples of the same herbages recovered from pen‐fed sheep and cattle oesophageal fistulates, were digested in vitro using rumen liquor. Measurements were made also of faecal nitrogen concentration (FN) and of indigestible acid‐detergent fibre (IADF) using the samples from the original trial. The range of in vitro disappearance values of the herbages (IVOMD; 0809‐0278) was slightly wider than those of in vivo digestibility (OMD; 0‐796‐0‐37I). Differences between OMD and IVOMD were greatest at low quality and relationships were best described by two separate linear regressions of OMD on IVOMD for (a) ryegrass, white clover, Agrostis‐Festuca and Nardus (RSD; 0 0185) and, (b) Molinia, Tricho‐phorum and Eriophorum (RSD; 00246). In vitro disappearance values of extrusa were higher than those of herbages offered by 00503 and 00156 units for sheep and cattle respectively, partly because of greater levels of ensalivation, especially in the sheep. The relationships of OMD to IADF and to FN were poor, though inclusion of faeces output and fitting parallel lines for communities in the regression of OMD on FN reduced the RSD to 0020. We conclude that in vitro digestion of samples of extrusa, using appropriate standards, is the best method of diet digestibility prediction for sheep and cattle grazing these communities.